Electronic and optical systems typically include a variety of electronic or optical components assembled on a substrate. For example, integrated circuits, resistors, capacitors, discrete transistors, inductors, voltage regulators, and electrical connectors are often mounted together on printed-circuit boards. As electronic and optical systems become smaller and more highly integrated, ever-smaller components must be assembled on substrates. For example, surface-mount components as small as 400 microns in length can be assembled on circuit boards using pick-and-place equipment.
Electronic and optical components are usually constructed on wafers using photolithographic methods and materials. Different wafer materials are well-suited for different device types, for example silicon wafers are used to make digital integrated circuits and sapphire or SiC substrates are commonly used with light-emitting diodes. Individual components can be removed from their native substrate, for example by dicing the wafer, packaging the components, and assembling the packaged components on a printed circuit board. However, such components and methods are most useful with relatively large electrical devices, for example greater than 200 microns, 500 microns, or 1 mm in a dimension.
Alternatively, devices such as light-emitting diodes (LEDs) are removed from their native substrate by laser lift off, for example as described in Large-area laser-lift-off processing in microelectronics, by Delmdahl et al. in Physics Procedia 41 (2013) pp. 241-248. This work describes UV laser lift-off delamination using 248 nm excimer laser sources to remove GaN LEDs from sapphire substrates. These devices must still be placed after lift-off.
Flexible electronic systems typically include a variety of electrical devices, either rigid or flexible, disposed on a flexible substrate. The flexible substrates can be very thin, for example less than 200 microns thick. Since equipment used in fabrication, for example photolithographic processing equipment, often relies on flat, rigid substrates in their manufacturing processes, one approach to making flexible electronic equipment relies on temporarily adhering a flexible substrate to a rigid substrate carrier, for example as also described in Delmdahl et al. in FIG. 3. After the processing and construction is completed, the rigid substrate carrier is removed or separated from the flexible substrate, leaving the electrical devices on the flexible substrate. However, for very small electrical devices, for example less than 200 microns in a dimension, it can be difficult to align and adhere the electrical devices with sufficient alignment accuracy on the flexible substrate or on an adhesive layer provided on the flexible substrate. For example, some photolithographic processes require heat and the coefficient of thermal expansion for a flexible substrate such as PEN or PET is much higher than the coefficient of thermal expansion of rigid substrates such as glass, quartz, silicon, or ceramics, making alignment much more difficult on the flexible substrate, especially for larger substrates such as those found in displays. Such rigid substrates can be four to twenty times more dimensionally stable than flexible substrates such as plastics.
In another approach to making flexible electronic devices, a flexible substrate is adhered directly to a native semiconductor wafer comprising the electrical devices, for example as described in Delmdahl et al. in FIG. 7 for a metal wafer. The electrical devices are removed from the native semiconductor wafer, for example by grinding the native semiconductor wafer, etching, or laser ablation. However, this approach limits the variety of electrical devices on the flexible substrate to those found on a single semiconductor wafer.
Small micro-devices having dimensions less than 100 microns, for example, can be assembled using micro-transfer printing techniques. For example, U.S. Pat. No. 8,722,458 describes transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate using a patterned elastomer stamp whose spatial pattern matches the location of the semiconductor elements on the wafer substrate. Small integrated circuit chips or chiplets are typically formed on a native silicon substrate using photolithographic processes. The silicon substrate facilitates the formation of anchors on the wafer and tethers between the wafer and the chiplet that are broken or separated during an exemplary micro-transfer printing process.
Micro-transfer printing can be used with a wide variety of component types in a wide variety of electronic and optical system, including processors, sensors, and energy emitters such as light-emitting diodes (LEDs). For example, CMOS devices are typically formed in silicon wafers, high-power transistors are often made using compound semiconductors such as gallium arsenide, and light-emitting devices such as light-emitting diodes are constructed in doped compound semiconductors such as gallium nitride, gallium phosphide, or gallium arsenide. These various components require a corresponding variety of materials and processing methods for making micro-transfer printable devices that can be directly micro-transfer printed from a native source substrate or wafer to a destination substrate. Devices that are not directly micro-transfer printed from a native wafer can be alternatively bonded to a handle wafer (for example as taught in U.S. Pat. No. 8,934,259) or transferred using two stamp transfer steps (for example as taught in U.S. Pat. No. 8,889,485).
There is a need, therefore, for methods and materials for constructing flexible electronic devices comprising a variety of different small electrical or opto-electronic devices at a high resolution and with improved accuracy.